Flexible high performance microbolometer detector material fabricated via controlled ion beam sputter deposition process

ABSTRACT

A microbolometer film material VOx having a value such that the thermal coefficient of resistance is between 0.005 and 0.05. The film material may be formed on a wafer. The VOx material properties can be changed or modified by controlling certain parameters in the ion beam sputter deposition environment. There is sufficient control of the oxidation process to permit non-stoichometric formation of VOx films. The process is a low temperature process (less than 100 degrees C.). Argon is used for sputtering a target of vanadium in an environment wherein the oxygen level is controlled to determine the x of VOx. The thickness of the film is controlled by the time of the deposition. Other layers may be deposited as needed to form pixels for a bolometer array.

BACKGROUND OF THE INVENTION

[0001] The present invention pertains to microbolometer sensors andparticularly to detector material for microbolometers. Moreparticularly, the invention pertains to a particular detector materialwhich is fabrication from a special ion beam sputter deposition process.The U.S. Government has certain rights in the present invention.

[0002] A major factor in the sensitivity of a bolometer is the TCR(thermal coefficient of resistance) of the detector material. Theoverall NETD sensitivity of the bolometer also depends on the noiselevel. Previous bolometer materials are typically high TCR metals with aTCR in the range from 0.003 to 0.004. These materials have low noise butalso have low TCR. Since the metals are reflectors, they also degradethe absorbance properties of the detector. Materials which undergo aphase transitions (i.e., Mott transition) can have a very high TCR's inthe transition region but can suffer from a number of problems. First,the latent heat accompanying the phase change for these materials maysignificantly decrease the sensitivity of the detector. Second, mostswitching material can be produced in only one form without additionaldoping, which defines the material resistance and TCR. Further, thetemperature range over which the transition occurs is typically verysmall requiring tight temperature control of the operation. Finally, thefilms must be produced in crystalline form which requires hightemperature depositions.

SUMMARY OF THE INVENTION

[0003] The present invention is peculiar vanadium oxide (VOx/ABx) (i.e.,VO_(x)/AB_(x)) detector material and process that is used to make thatmaterial. The x of VOx is a value fitting for the pixel being sputterdeposited by the present process and is not necessarily a specific digitsuch as “2”, but may be between 1 and 2.5. That material is deposited aspart of a pixel for a high performance microbolometer. The material isdeposited by an ion beam sputtering with control of the depositionprocess leading to a flexible detector process for microbolometerdetectors. These detector materials have optical, electrical, andthermal properties compatible with high performance detectors but whichcan be readily modified to suit individual requirements of an arraydesign.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004]FIG. 1 reveals a TCR comparison of various materials, includingVOx (ABx).

[0005]FIG. 2 is a graph showing electrical characteristics of a high TCRVOx detector film.

[0006]FIG. 3 exhibits determination of VOx resistance by control of thedeposition environment.

[0007]FIGS. 4a and 4 b are a schematic of the deposition system.

[0008]FIG. 5 is a schematic of the deposition process flow.

[0009]FIGS. 6a and 6 b show several stages of the deposited wafer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0010] VOx deposited by a controlled ion beam sputtering process hasproduced bolometer detector films with a wide range of performanceproperties which has led to a flexible detector manufacturing process.Some of the advantages of the present detector film materials of theinvention which is the class of these materials and the processing usedto make these materials, are noted. Bolometer film materials with highTCR's ranging from 0.005 to >0.03, such as 0.05, have been demonstrated.For a given resistivity, these materials are higher performing inmicrobolometers than any known material except for a few crystallinematerials (which have not been produced on pixels), as shown in FIG. 1.FIG. 1 is a graph showing a comparison of the TCR versus conductivity ofvarious materials relative to ABx (i.e., VOx). This class of materialshas a well behaved relationship between resistance and TCR given by therelationship TCR=A+B*log((rho) which, for example, may be (TCR(0.03+0.01*log((rho)). A wide range of pixel resistance and TCRproperties are possible by using different resistivity materials(obtained by slight modifications to the deposition process) combinedwith different detector patterns and thicknesses. These electricalproperties for any particular VOx film are well behaved andcharacterized over a wide range of temperatures and are not limited to anarrow transition region, as indicated by FIG. 2. For any particular VOxfilm, resistance is given by LnR=A+(B/T) and the TCR which is defined as$\frac{1}{R}\frac{R}{T}$

[0011] is therefore TCR=B/T². FIG. 2 exhibits the electricalcharacteristics of a high TCR VOx detector film. These characteristicsreveal a material having a well-behaved operation over a widetemperature range. Resistance levels are in the proper range to permithigh current bolometer operation for optimal responsivity. The films areamorphous and exhibit no latent heat effects unlike the phase transitioneffects in VO2. The films are stable after annealing if not taken to ahigher temperature. The resistance change on annealing is well-definedand can be corrected for by changes to the initial depositionconditions. 1/f noise levels are defined by V_(noise)=V_(bias){squareroot}{square root over (k/f)}. k values as low as 10⁻¹² to 10⁻¹⁴ make1/f noise contributions to total noise very small. Noise levels areclose to Johnson noise limited values.

[0012] The optical properties of VOx are compatible with high absorbancein the detector. The thermal mass of VOx, the thermal property ofimportance, is comparable to the major pixel material, Si3N4 (i.e.,Si₃N₄). These VOx films have a high TCR over a range of thicknesses fromas low as a few hundred Angstroms to as thick as 1500 Angstroms. Thismaterial of the films is compatible with microbolometer properties. TheVOx material properties can readily be modified by a simple change inthe ion beam sputter deposition environment of the process of thepresent invention, as revealed by FIG. 3. FIG. 3 is a graph that showsdetermination of VOx resistance by control of the ion beam sputterdeposition environment such as the gas control level. The present ionbeam sputtering provides sufficient control of the oxidation process topermit non-stoichiometric formation of VOx films. In other reactivedeposition techniques, the oxidation process tends to proceed tocompletion forming only stoichiometric material. The ion beam sputterdeposition is a lower temperature deposition process. This means thatadded flexibility in the patterning of VOx films can be achieved vialiftoff processing which entails the use of photoresist duringdeposition.

[0013] The method of the present invention is a process 18 of FIG. 5,performed in conjunction with deposition 10 of FIG. 4a, which is capableof making the above-note VOx material. Circular silicon wafers 11 withsubstrates containing electronic circuits and pixel lowers layers 12which are coated with an approximately 5000 Angstrom Si3N4 layer 13, areloaded into a five-wafer carousel 19 (of FIGS. 4a and 4 b) through port15 of deposition apparatus 10. Also, photoresist 74 may be on wafer 17which defines pixels. At present, each wafer 11 has a diameter 14 offour inches. Wafer 11 loading is step 16 of process 18 flowchart. System10 is first calibrated according to calibrate step 20 of process 18,which involves pumping the system down to 2×10⁻⁷ Torr pressure. Thevacuum system is capable of pumping to base pressure less than 1×10⁻⁷Torr with a throughput of better than an eight inch two-stage cyropump23 via valve 24. System 10 is pumped down by opening valve 21 which isleft open during the process. system 10 may be warmed up at step 25 withlamps 23. A residual gas analyser (RGA) 22 is warmed up at step 26before the process calibration. RGA 22 is connected to chamber 27 andhas a quadrupole probe which has an electron multiplier (EM) detector ina turbo-pumped sampling manifold to allow for operation in the 10-7 Torrpressure environment. RGA 22 may be connected to either upper chamber 31lower chamber 27. RGA 22 has an analog output.

[0014] Silicon wafers 11 may be anything with active electronics on itsuch as CMOS or bipolar devices 73 of FIG. 6, or other kinds of detectorcomponents up to the Si3N4 layer 13, photoresist 74 and/or VOx layer 35deposition. RGA 22 valve 28 is opened and argon flow is set toapproximately 3 scc/m at stage 29 of process 18. RGA 22 is calibratedvia an adjustment of its gain to get a standard RGA argon reading atstep 30. RGA 22 is a closed source device. It is connected to chamber 27in FIG. 4a. The output of RGA 22 detects a spectrum of species of gasconstituents and the detected gases are identified. RGA 22 is pumped outso that the pressure in RGA 22 is lower than chamber 27, and the ionspecies have to go through a small orifice at valve 23. RGA 22 is madeby UTI Inc. With RGA 22 valve open, the argon gas flow reading is setfor calibrating RGA 22 to a known argon peak level.

[0015] At step 32, the argon gas flow is set for gun 33 and hollowcathode neutralizer (HCN) 34. Gun 33 uses argon as a sputtering gas. Ionbeam 35 is neutralized by beam 40 of HCN 34. The ion gun is from IonTech Inc.

[0016] A target on target holder 36 is positioned relative to gun 33beam 35 so that vanadium is selected as a target material 46. The otherside of target holder has SiO2 as a selectable target material 47. Thetarget material is set at a 45 to 55 degree angle relative to thedirection of beam 35. The center of the target material is at a distanceof 7 and ⅝ inches from the beam 35 exit of gun 33. Argon gas flow to gun33 is set with MFC 37 to a 2.5 scc/m operating level, and to HCN 34 setwith MFC 39 to a 3.5 scc/m operating level. Xeon (Xe) may also be usedin place of Argon (Ar). Cooling water from source 41 is turned on to gun33 via line 42 and to target holder 36 via line 43. At step 38, an iongun power supply 44 is turned on and the initial gun start-up parametersare set to 20 mA at 1 kV. The ion gun source is turned on and the sourcestabilizes. Beam 35 of ion gun 33 is turned on. The power supplyvoltages are adjusted. There is plasma in gun 33 and ion beam 35 isgenerated or accelerated through grids of gun 35.

[0017] At next step 48, target material 46 is pre-sputtered with nooxygen. Pre-heat system quartz lamps 45 are turned off if on. Targetmaterial 46 is pre-sputtered for 240 seconds at the low power of 20 mAat 1 kV. During the next 240 seconds, target 46 is presputtered at themedium power of 35 mA at 1.5 kV. Presputter continues for the next 120seconds at the high power of 50 mA at 2 kV, which concludes systemcalibration stage 20. The presputter without oxygen is for cleaningtarget 46 for ten minutes or so.

[0018] The following stage 50 begins with step 49 of presputter with anoxygen ramp to condition target 46 to a desired RGA 22 level. The oxygengoes up by little steps and then to larger steps as one measures theoxygen within system 18. This not cleaning target 46 but conditioningtarget 46 to the desired RGA 22 level. One increases the oxygen to get acharacter profile for film 35 based on past experience and RGA 22 is setat an arbitrary level called y. The beam 35 power is kept at 50 mA at 2kV and controller 53 of MFC 51 sets the oxygen flow to chamber 27 viatube 52, to 0.5 scc/m for 90 seconds. Then the oxygen flow is increasedby 0.1 scc/m for 90 seconds. The latter is repeated until the 32 AMUpartial pressure increase is equal to or greater than ten times over theprevious partial pressure. The operating level setpoint is based on thepartial pressure rise of the previous step which is about midpoint ofthe last 32 AMU partial pressure increase. The precise location of theoperating setpoint determines the resistance and TCR. The 32 AMU partialpressure setpoint is entered into controller 53, which determines thelevel of oxygen flow for VOx film 35 deposition. One increases the flowof oxygen in an incremental way until O₂ is at a point where the RGAoutput an O₂ signal. One measures the mass and monitors the RGA O₂. Iongun 33 is run at a set level with a fixed voltage and current.Monitoring of RGA 22 of 32 AMU is done at controller 53 where the flowis adjusted to achieve the starting level. A computer processor 56 maybe used at step 59 to monitor RGA 22 and adjust the oxygen flow viacontroller 53 to achieve starting condition or level.

[0019] At step 54, rotation of wafer substrate 11 is started. A controlloop is started with a presputter for 300 seconds at the setpoint ofcontroller 53. A shutter 55 is opened at step 58 after system 10 hasstabilized. A timer 57 is started with the time determined by a desiredthickness, of which the deposition rate is approximately at 25 Angstromsper second. RGA 22 may be monitored and oxygen flow adjusted at step 60during deposition step 61. The center of sputtered target 46 with itssurface at 45 to 55 degrees relative to and 12 inches from theto-be-deposited surface of wafer 11, is aligned at one inch from thecenter of wafer 11. After the desired thickness is achieved, thenshutter 55 is closed at step 62. Then carousel 19 is turned at step 63for the next wafer 11 to be coated, the control loop starts with thepresputter at step 49 of deposition stage 20, and goes through the samesteps of the process for the previous wafer 11 deposition. After all thesubstrates 11 of carousel are deposited, the control loop of stage 20 isstopped. The oxygen MFC 51 is set to zero scc/m, RGA 22 sample valve 28is closed, ion beam 33 is turned off or to stand-by mode, and RGA 22 isturned off.

[0020] Process 10 is a low temperature process which does not go over100 degrees C. which would cause photoresist 74 to harden. Typicallythis process is performed at about 80 degrees C. or less.

[0021] Process 18 for wafers 11 moves on to stage 64 for SiO2deposition. At step 65, target holder 36 is rotated so that the surfaceof target 47 will be at a 45 degree angle relative to the direction atthe center of ion beam 35 when it is turned on, and the oxygen flow isset to 2.0 scc/m at MFC 51. First wafer 11 is rotated in by carousel 19as step 66. Ion gun 33 is turned on at 50 mA at 2 kV to presputtertarget 47 for 300 seconds. Shutter 55 is opened and timer 57 is started.Timer 57 is set to a time period to attain a desired thickness of SiO2on wafer 11 at a deposition rate of 0.33 Angstrom per second at step 67.Then shutter 55 is closed. Next wafer 11 is rotated to by carousel 19for SiO2 deposition at step 68, and steps for depositing SiO2 onprevious wafer 11 are followed. After the last wafer 11 is coated withdeposition of SiO2, the system 10 is shut down at step 69 of stage 64.Ion beam 35 is turned off, MFC 51 is set to 0.0 scc/m, ion beam source33 is turned off, and substrate rotator 70 and carousel 19 are turnedoff. The one should wait and let system 10 cool down for 45 minutes.After cool-down, Hivac valve 24 is closed. and system 10 is vented withdry N2 from supply 71 through valve 72. One may open system 10 when itis at atmospheric pressure and remove wafers 11. To start process 18over with another set of substrates or wafers 11, one introduces wafers11 into upper chamber 31 through port 15 and close system 10. Quartzlamps 45 are turned on to a preset level to yield 80 degree C.temperatures in system 10. N2 vent gas valve 72 is turned off and MFC 37and MFC 39 are set to 0.0 scc/m. Next pump down with cyropump 23 andopen Hivac valve 24. Check for leaks and go through process 18 asindicated above. The values of the parameters and settings of theabove-noted embodiments are by example only, but could vary from case tocase.

[0022] From wafer 11 processed by system 10, is made microbolometerpixels 77 of FIGS. 6a and 6 b. On wafer 11, prior to system 10depositions of VOx and SiO2, may be Si substrate 12 covered with adevice layer 73. On layer 73 is pixel Si3N4 material layer 13. Pixels 77are defined with a photoresist mask 74. Then, VOx layer 35 and SiO2layer 75 are respectively deposited on wafer 11 as indicated above andrevealed in FIG. 6a. The next step is to chemically remove photoresist74 and a portion of layers 35 and 75 formed on and over photoresistlayer is likewise removed, resulting in pixel 77 as shown in FIG. 6b. Avia or hole 78 is etched through layer 75 for electrical contact whichis made with a nichrome (NiCr) strip 76 formed by depositing andpatterning on a small portion of pixel 77. NiCr 76 forms the contact tothe VOx portion of pixel 77. VOx layer 35 covers a major portion ofpixel 77. An additional Si3N4 layer may be formed on pixel 77 of FIG. 6bfor more protection. Layer 75 is about 200 Angstroms. If layer 75 werenot put on prior to placement of contact 76, then the VOx portion of thepixel would be degraded due to the electrical degradation of VOx duringpresputter of the film prior to Si3N4 and subsequent NiCr deposition ofstrip 76. Layer 35 may range from 200 to 2000 Angstroms depending on thedesired TCR. Layer 13 is about 500 Angstroms. But is an Si3N4 layer 80is formed on layer 75, then NiCr strip would go through layer 80 andlayer 75 to contact VOx layer 35.

We claim:
 1. A method for depositing flexible high performancemicrobolometer detector material comprising: loading at least one waferinto a chamber; pumping down the chamber to a vacuum; setting a flow ofargon into the chamber at a certain rate; calibrating a residual gasanalyzer (RGA) so that the RGA can detect a spectrum of species ofgases; setting the flow of argon gas for an ion gun wherein the argon isa sputtering gas; positioning a target of vanadium proximate to the iongun; presputtering the target of vanadium; set a flow of oxygen into thechamber for presputtering; set the flow of oxygen into the chamber to alevel that is adjusted at least partially with an RGA indication;activating ion gun to sputter with argon ions, vanadium atoms off of thevanadium target which combine with a certain portion of oxygen atoms indeposition on the at least one wafer, the portion of oxygen atomsdetermined by a setting of the flow of oxygen into the chamber; andsetting a timer for determining the duration of the deposition to attaina certain thickness of deposited material on the at least one wafer. 2.The method of claim 1 wherein the deposited material is VOx where x is avalue such that a thermal coefficient of resistance (TCR) is between0.005 and 0.05.
 3. The method of claim 2 wherein VOx has a log ofconductivity between zero and three.
 4. A VOx material comprising:vanadium and oxygen in a portion of VOx where x is a value such that athermal coefficient of resistance (TCR) is between 0.005 and 0.05; andwherein the VOx has a log of conductivity between zero and three.
 5. TheVOx material of claim 4 wherein the relationship between its resistanceand TCR is provided by the relationship TCR=A+B*log(rho).
 6. The VOxmaterial of claim 5 wherein: A is between 0.005 and 0.05; and B isbetween 0.1 and 0.001.
 7. A pixel comprising: a silicon substrate; adevice/sacrificial layer formed on said silicon substrate; a Si3N4 layerformed on said device/sacrificial layer; a VOx layer formed on saidSi3N4 layer; an SiO2 lay formed on said VOx layer; and an Si3N4 layerformed on said SiO2 layer.
 8. The pixel of claim 7 wherein x of VOx is avalue such that a thermal coefficient of resistance (TCR) is between0.005 and 0.05.
 9. The pixel of claim 8 wherein the pixel is one of aplurality of pixels that are of a bolometer array.